This invention relates generally to a method and device for generating fluid droplets, and more particularly relates to a method and device for generating droplets of immiscible fluids using focused acoustic ejection technology.
In many fields, it is often desirable to generate droplets of a fluid mixture composed of components that normally are immiscible with one another, wherein one or more of the components may be quite viscous. However, with prior methods for generating droplets of viscous fluids, control over droplet size is difficult. For example, conventional inkjet technologies such as piezoelectric and thermal inkjet printing are limited by the need to force a viscous material through a small nozzle. In order to dispense quantities of fluid on the order of 1 picoliter in volume, nozzle openings with dimensions of under 30 microns would be required. The energy required to move a viscous fluid out of such a small nozzle opening would be very high and would result in substantial shearing of the fluidic material. Nozzle clogging is also problematic with these printing technologies.
For a thermal inkjet printer, the ejection energy comes from the vaporization of the fluid to be printed. Most viscous fluids have an extremely high boiling point and would require significant thermal energy input. Piezoelectric printing might be a more efficient way of ejecting droplets of viscous material, but the thermal energy required would still be substantial. For example, U.S. Pat. No. 5,229,016 describes a method for dispensing solder with a piezoelectric ejection device. The system requires elevated temperature and a backpressure system of 30 psi in order to eject solder through a 25 xcexcm orifice. Even with pressure assist, the maximum ejection rate for solder is on the order of 10 kHz. In U.S. Pat. No. 5,498,444, a method is described for ejecting polymers using a piezoelectric ejection device with the device operating at up to 40 cps along with elevated temperatures. The need for elevated temperatures, of course, reduces the number of materials one can work with, as heating many materials in order to reduce viscosity can result in degradation. Other devices for producing droplets of fluids, such as described, for example, in U.S. Pat. No. 4,812,856 to Wallace et al., are disadvantageous in other respects as well, including slow repetition rate due to refill time.
The use of acoustic energy in printing technology is known. For example, U.S. Pat. No. 4,308,547 to Lovelady et al. describes a liquid drop emitter that utilizes acoustic principles in ejecting liquid from a body of liquid onto a moving document for forming characters or bar codes thereon. Lovelady et al. is directed to a nozzleless inkjet printing apparatus wherein controlled drops of ink are propelled by an acoustical force produced by a curved transducer at or below the surface of the ink. In contrast to inkjet printing devices, nozzleless fluid ejection devices as described in the aforementioned patent are not subject to clogging and the disadvantages associated therewith, e.g., misdirected fluid or improperly sized droplets.
However, the development of nozzleless fluid ejection has generally been limited to ink printing applications. Since development of ink printing applications is strongly influenced by economic concerns, the bulk of the development efforts have been concentrated on reducing the cost of printing rather than on improving the quality of printing. In other words, development efforts for acoustic printing have focused on improving the speed of printing rather than accuracy. For example, U.S. Pat. No. 5,087,931 to Rawson is directed to a system for transporting ink under constant flow to an acoustic ink printer having a plurality of ejectors aligned in an axis, each ejector associated with a free surface of liquid ink. Having a plurality of ejectors generally increases printing speed. However, it is more difficult to control fluid ejection, specifically droplet placement, when a plurality of ejectors is used in place of a single ejector.
As another example, U.S. Pat. No. 4,797,693 to Quate describes an acoustic ink printer for printing polychromatic images on a recording medium. The printer is described as comprising a combination of a carrier containing a plurality of differently colored liquid inks, a single acoustic printhead acoustically coupled to the carrier for launching converging acoustic waves into the carrier, an ink transport means to position the carrier to sequentially bring the differently colored inks into alignment with the printhead, and a controller to modulate the radiation pressure exerted against the inks. It is disclosed that this type of printer is designed to allow the realization of cost saving. However, this device can eject only a limited quantity of ink from the carrier before the liquid surface moves out of acoustic focus and drop ejection ceases.
Thus, there is a need in the art for improved system that makes use of focused acoustic ejection technology to generate droplets of immiscible fluids without the disadvantages associated with photolithographic techniques, inkjet printing devices relying on a nozzle for droplet ejection, and prior acoustic ejection systems.
Accordingly, it is a primary object of the invention to address the aforementioned need in the art by providing a novel method and device for generating droplets of immiscible fluids using focused acoustic ejection technology.
It is another object of the invention to provide a method for generating a droplet of at least two immiscible fluids, wherein focused acoustic energy is applied to a reservoir containing the immiscible fluids in a manner effective to eject a droplet of fluid from the reservoir.
It is still another object of the invention to provide such a method wherein at least one of the immiscible fluids has a viscosity of at least about 10 cps.
It is yet another object of the invention to provide such a method wherein a first fluid is aqueous, a second fluid is nonaqueous, e.g., lipidic, and one of the two fluids contains a biomolecule or a pharmaceutical agent.
It is a further object of the invention to provide such a method wherein the reservoir is maintained at an elevated temperature, such that a first fluid in the reservoir comprises a liquid metal or a superconducting alloy and a second fluid comprises a wax or glass.
It is still a further object of the invention to provide such a method wherein the droplets containing the immiscible fluids are ejected onto localized regions within discrete sites on a substrate surface.
It is an additional object of the invention to provide a device for acoustically ejecting immiscible fluids from each of a plurality of fluid reservoirs, the device comprising an acoustic ejector constructed from an acoustic radiation generator and a focusing means, and a means for positioning the ejector in acoustic coupling relationship to each of the reservoirs.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.
In one aspect, then, a method and device are provided for generating droplets of immiscible fluids using focused acoustic energy. The use of focused acoustic energy as implemented herein enables generation of extremely fine droplets, on the order of 1 picoliter or less, with extraordinarily accurate and repeatable droplet size and velocity. The method involves use of a device comprising: one or more reservoirs each containing two or more immiscible fluids; an ejector comprising an acoustic radiation generator for generating acoustic radiation and a focusing means for focusing the acoustic radiation generated; and a means for means positioning the ejector in acoustic coupling relationship to each of the reservoirs.